1. Field of the Invention
The present invention is related to traps for removing reactants from reaction chamber effluents, and more specifically to a method and apparatus for removing reactants in effluents of atomic layer deposition and other processes before they can damage vacuum pumps.
2. State of the Prior Art
Atomic layer deposition (ALD) of thin-film materials in semiconductor device fabrication, such as thin barrier layers, high dielectric constant (high K) layers for capacitors and gate dielectric layers, and the like, has a number of advantages. For example, ALD processes typically have low particle content and relatively low processing temperatures, and ALD produces high quality films with uniform film thicknesses and has the ability to deposit such materials in deep trenches in the substrate materials. However, a number of practical problems in its implementation have inhibited the application of ALD in full scale commercial production of semi-conductor devices and other thin film applications. Recent technological developments aimed at solving these problems, for example, valves capable of more precise control of feed gases to enable increases in rates of deposition are contributing to development of larger scale ALD processes and equipment. However, such larger scale ALD uses have revealed or lead to additional problems that have to be solved in order for ALD to become economically viable for fabrication of thin-film semiconductor materials on a large scale basis.
One of those problems is that because of the inherent nature of ALD with its deposition of one distinct atomic layer at a time with successive flows of reactants separated by short pulses of purge gas. During typical ALD processes, two chemical precursors (A and B) are required to react with each other to form, i.e., deposit, the solid thin film on the wafer (substrate) surface. This process actually involves at least four steps to complete a deposition cycle. First a reactant or precursor A is fed into the ALD process chamber to allow the formation of a monolayer of A molecules on the surface of the substrate by physical adsorption. Then, purge gas (typically inert) is fed into the process chamber to remove the precursor A molecules in gas phase, which eliminates a major source of particles on the wafer (substrate) surface, because gas phase chemical reaction is often the major source of particles on the wafer surface. Next, the precursor B is fed into the process chamber to start the deposition reaction process. Since there are very little, if any, reactive A molecules in the gas phase, chemical reactions occur primarily on the surface of the wafer because of the limited supply of the precursor A. Finally, additional purge is performed before the next deposition cycle starts. It is very clear that large amounts of the reactant gases—often greater than 80 to 90 percent—flow out of the reaction chamber in the effluent. Such large amounts of residual reactant gases in the ALD process effluents cause severe wear and damage to the vacuum pumps, where relatively high temperatures produced in the pump cause them to react together to form highly abrasive particles. In some ALD systems, for example, those used for depositing aluminum oxide (Al2O3) from trimethylaluminum Al(CH3)3 and an oxidant or those used to deposit hafnium oxide (HfO2) from hafnium chloride (HfCl4) and an oxidant, the vacuum pumps can become so damaged within a few hours to a few days of operation that they have to be replaced.
Therefore, there is a very significant need for new methods and apparatus for protecting vacuum pumps in ALD systems that are especially suited to the unique characteristics of such ALD reactants and processes.